The immunoglobulin heavy chain (IgH) locus is subject to many modifying events over the course of B cell development and differentiation into Ig-secreting cells. These events include assembly of variable region (VH) coding sequences, activation of the IgH promoter, heavy chain class switch recombination (CSR), somatic hypermutation, and a shift from membrane to secreted heavy chain production. The focus of this application is the 3' region of the murine IgH locus, a region that contains several transcriptional enhancers whose functions in these various processes remain largely undefined. This region plays an important role in murine myeloma (Ig-secreting) cells, both with respect to IgH gene expression and with respect to activation of the oncogene c-myc. The latter is a central event in the malignant transformation of murine cells and also in the transformation of human Burkitt lymphoma cells. In this proposal, we will further explore the functions of the 3 'IgH enhancer region, using two different model systems. One is a mini-locus system that we have recently developed, which affords an analysis of these enhancers' effects on gene transcription in the context of chromatin. This provides us with a means for testing the hypothesis that some, but not all, of these enhancers serve a chromatin-remodeling function that is B cell-stage limited. As another model system, we are developing a bacterial artificial chromosome to study enhancer function both in cell lines and in mice. This system will allow us to study the developmental pattern of enhancer activity and will provide information and materials that will eventually allow us to study enhancer control of processes such as CSR and somatic hypermutation. A final aim in this application is to ask whether, subsequent to VH gene assembly, the IgH locus functions normally in the absence of the intronic enhancer E mu. By this means, we can identify functional differences and redundancies among the control elements regulating this complex locus.